Hydrothermal synthesis of ZnO@polysiloxane microspheres and their application in preparing optical diffusers

Abstract

In this study, optical diffusers based on ZnO@polysiloxane microsphere fillers were successfully prepared by a solvent-free UV curing process. ZnO@polysiloxane microspheres were synthesized via a highly efficient and facile hydrothermal synthetic method. The microscope analysis revealed that the ZnO@polysiloxane microspheres were about 6–8 μm in diameter. The optical properties of the diffusers were also measured for the first time. The results suggest that the novel diffuser based on ZnO@polysiloxane microsphere fillers possessed suitable light transmittance, good diffusion capacity, and low incident angle dependence. In addition, its tunable optical diffusing capacity relied on the amount of ZnO@polysiloxane microsphere fillers. When the concentration of the ZnO@polysiloxane microspheres was as high as 15 wt%, the diffusing properties of the novel diffusers reached the level of a commercially available diffuser. Therefore, the new fillers can be widely used for the preparation of multifunctional diffusion materials such as touch-panel functions, monitors, military projectors, and TVs.

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1. Introduction

Liquid crystal displays (LCDs) require better light diffusion for their development to become larger and thinner.¹ The optical diffuser is the key component in the backlight unit (BLU) of LCDs, which plays the role of smoothing the illumination profile behind the LCD, making it more uniform.^2,3 In early studies, the properties of optical diffusers were enhanced through the careful design of the diffusion particles and manufacturing conditions.^4,5 Quite recently, the development of sophisticated surface patterning techniques has made researchers to explore the feasibility of improving the light diffusing effect of diffusers by patterning optical micro-structures on the upper surface of the film.^6–8 Besides, our group have proposed hybrid optical diffusers in which various functions were combined.^9,10

Although the cost of the prepared diffusers by coating organic light-diffusing particles was relatively low, there were still drawbacks with the coated optical materials, including the dispersion of particles, low thermal stability, single function, and complex process for the preparation of particles.^11–15 In particular, because the progress in the research of optical diffusers was mainly protected in the form of patents, the studies, which were needed to be explored to get an early breakthrough, were very limited.²

Core–shell hybrid microspheres have become one of the most effective sources for fabricating advanced materials due to their special structure, which is different from common particles. Besides, the structure, size, and composition of the particles can be altered by a controlled synthesis route over a broad range to tailor a variety of novel properties (e.g., electrical, thermal, mechanical, optical, and magnetic).^16–22 The methods developed to produce core–shell microspheres include seed polymerization, macromonomer method, self-assembly method, and gradual heterogeneous condensation method.^23–25 However, these methods are not simple and convenient enough and cannot satisfy the demands of atom economics. Herein, we reported a facile, efficient, simple and green approach for obtaining ZnO@polysiloxane core–shell microspheres via a hydrothermal assembly process. The synthesis route is shown in Scheme 1. Moreover, novel diffusers based on the ZnO@polysiloxane fillers were prepared by a facile UV curing process. The optical properties and diffusing abilities of the materials were characterized to show their feasibility as optical materials. Besides, for the first time, a series of optical performances were investigated.

Scheme 1 The synthesis route of ZnO@polysiloxane core–shell microspheres.

2. Experimental procedures

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99.5%), urea (CO(NH₂)₂, 99.5%) and sodium tartrate (Na₂C₄H₄O₆·2H₂O, 99.5%) were obtained from Shanghai Chemical Reagent Company. We prepared poly(VTMS) microspheres ourselves via a sol–gel method (see ESI†). Tri(propylene glycol) diacrylate, photoinitiator 184, and polyurethane acrylate were obtained from Guangzhou Sadoma Chemical Co., Ltd PET films with a thickness of 50 μm were obtained from Shenzhen Shenli Optical Film Co., Ltd.

2.2. Preparation of ZnO@polysiloxane sample

In a typical synthesis process, 2 mmol zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), 4 mmol urea (CO(NH₂)₂) and 0.15 mmol sodium tartrate were dissolved in 70 mL deionized water to form a clear solution, and then 2 g polysiloxane microspheres (self-preparation, see ESI†) were added to the abovementioned solution with stirring for 10 min. The mixed solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 110 °C for 4 h. The precipitate was centrifuged and washed several times with deionized water and absolute ethanol, and then dried at 190 °C for 12 h. Thus, ZnO@polysiloxane microspheres were obtained.

A portion of the sample was then calcined at 300 °C for 2 h, to investigate the structure of the synthesized microspheres. To compare the structure of the samples by XRD analysis, ZnO particles were prepared by a hydrothermal method, as described in the literature.²⁶

2.3. Preparation of optical diffuser sample

Subsequently, complex films were prepared by solvent-free UV curing. 5 g tri(propylene glycol) diacrylate, 0.4 g photoinitiator 184, and 15 g polyurethane acrylate were mixed in the dark. Different concentrations of relative particles with 5–20 wt% were added to the abovementioned mixture. The film samples were obtained by double-sided coating in 50 μm PET. Then, each side of the film was placed under a 100 W UV lamp (365 nm) for 10 min. Finally, it was placed in an oven at 60 °C for 24 h. The commercial diffuser for comparison was obtained from Dong Xucheng Chemical Co., Ltd (DXC Diffusion 50-BDN).

2.4. Characterization

The structure of ZnO@polysiloxane was examined by FT-IR, powder transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), the results of which are shown in the ESI.† The UV-vis-NIR spectra of the optical diffusers were obtained on a Shimadzu UV 3600 spectrometer, and the sample was made in the form of a film via a precision coater. A transmittance haze meter (WGT-S) was used for measuring the properties of the optical diffusers. Optical diffusing properties were measured by a light intensity distribution measuring instrument (WGZ-III).

3. Results and discussion

3.1. Diffusing fundamentals

The light-diffusing phenomenon and its relative fundamentals are very important for the light-diffusing study and the design of light diffusion agents. Fig. 1 displays the diffusing effect based on diffusing microspheres. As can be seen, light-diffusing originates from scattering and refraction. The former contains Rayleigh scattering and Mie scattering, and the distinction between them is the size of the diffusing particles. When the diameter of the diffusing particles is considerably smaller than the wavelength of visible light, Rayleigh scattering occurs. Mie scattering appears in diffusing particles whose diameter is as much as the wavelength of visible light. In addition, the scattering effect decreases continuously with the increasing size of particles. The abovementioned analysis indicates that the size of the diffusing particles for optimal diffusers should be concentrated in the range of 1–10 μm. The latter mainly relies on the shape and internal structure of the particles, and a spherical structure can obtain excellent diffusing effects.⁹ Besides, the inner structure of the particles play a more important role in the light-diffusing effect than the size-distribution of diffusing particles. Compared to common spherical structures, the core–shell structure can create a better light-diffusing effect due to multi-scattering.

Fig. 1 Light-diffusing phenomenon and relative fundamentals.

3.2. XRD analysis

The X-ray diffraction (XRD) patterns of the synthesized composites are shown in Fig. 2. The XRD pattern (2θ > 25°) of the synthesized composite powders agreed well with the peaks of standard ZnO (Joint Committee on Powder Diffraction Standards (JCPDS) File Card no. 36-1451), which confirmed that the synthesized products contained ZnO. In addition, a small broad peak was formed in the hydrothermal product (shown in XRD curve (b)), which corresponds to the poly(VTMS) XRD pattern.

Fig. 2 X-ray diffraction (XRD) patterns of (a) bare ZnO; (b) the product prepared by hydrothermal method; (c) poly(VTMS).

Comparing the peak intensities of curve (a) with curve (b), it can be observed that the XRD pattern of the hydrothermal product presented weak ZnO peaks and poly(VTMS) peaks, which indicated that the structure of the product decreased in crystallinity. This also suggested that the composites of the hydrothermal product experienced a chemical assembly other than the physical blend of poly(VTMS) microspheres and formed ZnO particles.

3.3. SEM analysis

The dimensions and morphological features of the synthesized particles were observed by SEM, and the images are shown in Fig. 3. The poly(VTMS) particles were prepared via the sol–gel process, and their morphology was obtained as shown in Fig. 3(a). The particles exhibited a smooth regular spherical shape, and their sizes were mainly 5–8 μm. The morphology of the hydrothermal product is displayed in Fig. 3(b)–(d), which present a rough spherical shape. Fig. 3(c) and (d) are the enlarged view of the red circle in Fig. 3(b), which clearly displays the surface morphology of the target product. The size of the final product was about 5–8 μm, and the morphology of the product was a regular spherical shape. The abovementioned analysis suggests that the rough surface of the product would not obviously increase the thickness of the microspheres shown in Fig. 3(a).

Fig. 3 SEM photomicrographs of (a) poly(VTMS) microspheres; (b) ZnO@polysiloxane; (c) and (d) enlarged view within the red circle of (b).

To investigate the internal structure of the microspheres, the calcined sample was measured by SEM (shown in Fig. 4). Fig. 4(b) is the magnified view of Fig. 4(a). As can be seen in the delineated boxes of Fig. 4, the broken structure of the final product obviously revealed the core–shell structure of the microspheres. Comparing the mentioned SEM photomicrographs, it was suggested that the inner layer of final product was composed of poly(VTMS) microspheres and the outer layer was composed of ZnO. In addition, the microscope analysis revealed that the core–shell microspheres were about 5–8 μm in diameter and with a shell thickness of approximately 400–800 nm. Therefore, the core–shell structure with a nanometer size shell layer gave the microspheres special physical and chemical properties, which are different from solid particles due to the difference in refractive index, larger specific surface area, and nanostructured wall; moreover, these materials have potential applications in controlled delivery and as artificial cells and light-weight fillers.

Fig. 4 SEM photomicrographs of (a) the comparison sample via hydrothermal process, which was then calcined at 300 °C for 2 h; (b) magnified view of (a).

3.4. Light diffusion phenomenon

The novel optical diffusers were examined to understand the effect of their diffusing ability for different concentrations of diffusing fillers. The quantity of ZnO@polysiloxane microspheres was modulated, including 5, 10, 15 and 20 wt%. A laser with a wavelength of 405 nm was used to illuminate the diffusers directly and the optical pattern was recorded by a digital camera (shown in Fig. 5). It was clear that the diffusion range gradually increased with the increasing concentration of ZnO@polysiloxane microspheres. Moreover, the prepared diffusers had good visible light transmittance, and their tunable optical diffusing capacity relied on the amount of ZnO@polysiloxane microspheres fillers. When the concentration of ZnO@polysiloxane microspheres was up to 15 wt%, the diffuser possessed good light-diffusing effects, which were similar to the commercially available optical diffusers.

Fig. 5 Diffusing images with a 405 nm laser.

3.5. Light diffusing properties

The transmittance and haze of new diffusers with different concentrations of ZnO@polysiloxane microspheres were measured by a transmittance haze meter. Fig. 6(a) shows the transmittance and haze of the diffusers. The transmittance of the diffusers slightly decreased with the increasing concentration of ZnO@polysiloxane microsphere, while the haze increased. When the concentration of the ZnO@polysiloxane microspheres was up to 15 wt%, the haze of the new diffusers exceeded that of the commercially available optical diffusers. Fig. 6(b) shows the relationship between the incident angle and scattering angle based on diffusing films with different concentrations of filler. It can be observed that the scattering angle, also named as scattering range, increased with the concentration of ZnO@polysiloxane microspheres. Due to multi-scattering derived from the ZnO@polysiloxane microspheres structure, the new diffuser presented low incident angle dependence. Thus, all different angles of incident light can be scattered through the new diffuser when the concentration of the novel diffusing fillers has a certain value. When the concentration of the ZnO@polysiloxane microspheres was up to 15 wt%, the light scattering properties of the new diffusers can reach the level of the commercially available optical diffusers.

Fig. 6 (a) The impact of transmittance and haze on the concentration of ZnO@polysiloxane microspheres; (b) the relationship between incident angle and scattering angle based on diffusing films with different fillers; (c) the related brightness for different ZnO@polysiloxane concentrations as a function of half a scattering angle.

Fig. 6(c) shows the related brightness as a function of half a scattering angle, wherein the related brightness is defined as the measured flux divided by the maximum flux. Thus, the percentage of the 0th order beam, defined as the flux in the center of the diffusing beam divided by the overall flux of a laser beam, was also evaluated. It was observed that the intensity of the 0th order beam decreased with the amount increasing concentration of ZnO@polysiloxane, which revealed that the scattering angle and the percentage of the 0th order beam could be modulated by varying the concentration of ZnO@polysiloxane microspheres. Therefore, various diffusing performances of optical diffusers can be obtained by adding a certain amount of ZnO@polysiloxane.

4. Conclusions

In conclusion, we explored an innovative and cost-effective optical diffusing filler with good optical properties. ZnO@polysiloxane microspheres were prepared by a facile hydrothermal method, and the diffusers based on their fillers can be manufactured via a solvent-free UV curing process. The optical properties of the new diffusers were also measured for the first time. The new diffuser based on the ZnO@polysiloxane microsphere fillers possessed suitable light transmittance, good diffusion capacity, and low incident angle dependence, which are critical and necessary for excellent optical diffusers. In addition, the tunable optical diffusing capacity relied on the amount of ZnO@polysiloxane microsphere fillers. When the concentration of the ZnO@polysiloxane microspheres was up to 15 wt%, the diffusing properties of the novel diffusers reached the level of a commercially available diffuser. Thus, ZnO@polysiloxane microspheres are considerably suited to act as optical fillers, and such microspheres can be widely used for the preparation of different multifunctional optical diffusers.

Acknowledgements

The authors are grateful to the financial supports of the National Natural Science Foundation of China (Grant no. 21306023, 21376051, 21106017 and 51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant no. BA2011086), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20100092120047), Key Program for the Scientific Research Guiding Found of Basic Scientific Research Operation Expenditure of Southeast University (Grant no. 3207043101) and Instrumental Analysis Fund of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12217h

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